Several readers (including Tracy Valleau and Nathan Green) report that, with GoMac 1.4.2 installed, Speed Doubler 8.0.1's Faster File Copying is disabled. "Removing the GoMac control panel restores the Fast Copy functions." Going back to Speed Doubler 8.0 also solves the problem.
This problem is now confirmed on the GoMac web page. Sam Caughron (of Proteron) writes: "It has yet to be determined which program is the cause of the problem. If the problem is with GoMac, there will be a fix available sometime early this coming week." (Latest word is that a forthcoming update to GoMac will fix this.)
Meanwhile, I have gotten an assortment of other bug reports related to Speed Doubler, but not enough confirming reports to want to post them yet.